My 92 ford f150 has been running hot?

For example the temp gauge reads to the letter "L" in the word normal on the gauge. I noticed this first happened when I had some mud looking sludge in my radiator. The truck would run hot while driving on the highway for no more that 15 minutes. Since then I've flushed the radiator, changed the water pump, I've even bypassed the heater core to make sure it wasn't clogged and causing the overheating. I have no visual leaks that I know of and I also have no thermostat as well. Fan seems to be working properly. Could I have a bad radiator or possibly need to put a thermostat back in the vehicle?

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